Method of making aluminium bearing alloy strip

ABSTRACT

An aluminium-lead bearing alloy, optionally containing tin, silicon, copper and cadmium, is produced in strip form by spraying droplets of the molten metal onto a substrate under conditions such that they strike the substrate in a highly undercooled condition and are chilled to solidification at the substrate at the rate of at least 103*C/second so as to precipitate the lead in the form of a fine dispersion of separate particles. The deposit of solidified droplets is then subjected to compaction. A surface layer of commercial purity aluminium or other suitable aluminium alloy may be applied to one or both surfaces of the aluminium-lead alloy strip to facilitate the production of a composite bearing alloy- steel strip by a rollbonding procedure.

i United States Patent [191 Baker et al. [4 1 Sept. 10, 1974 METHOD OF MAKING ALUMINIUM 3,742,585 7/1973 Wentzell 29/1493 s x BEARING ALLOY STRIP 3,775,156 11/1973 Singer 117/105 X [75] Inventors: William Albert Baker, New York,

N.Y.; John Arthur Wheatley, Adderbury West, England; Peter John Read, South Newington, England; Keith Graham Latimer,

Greatworth, England; Terence David Warren Reynolds, Daventry, England [73] Assignee: Alcan Research and Development Limited, Montreal, Quebec, Canada [22] Filed: June 14, 1973 [21] Appl. No.: 369,804

[30] Foreign Application Priority Data June 21, 1972 Great Britain 29172/72 [52] US. Cl. 29/149.5 S, 29/475, 29/5277, 117/105, 1l7/l05.3, 164/46 [51] Int. Cl B2ld 53/10 [58] Field of Search 29/1495 S, 149.5 PM, 475, 29/5277, 527.2; 117/105, 105.3; 164/46 [56] References Cited UNITED STATES PATENTS 3,670,400 6/1972 Singer 29/5277 X Primary Examiner-Charles W. Lanham Assistant ExaminerV. A. DiPalma Attorney, Agent, or Firm-Cooper, Dunham, Clark, Griffin & Moran [57] ABSTRACT An aluminium-lead bearing alloy, optionally containing tin, silicon, copper and cadmium, is produced in strip form by spraying droplets of the molten metal onto a substrate under conditions such that they strike the substrate in a highly undercooled condition and are chilled to solidification at the substrate at the rate of at least 103C/second so as to precipitate the lead in the form of a fine dispersion of separate particles. The deposit of solidified droplets is then subjected to compaction. A surface layer of commercial purity aluminium or other suitable aluminium alloy may be applied to one or both surfaces of the aluminium-lead alloy strip to facilitate the production of a composite bearing alloysteel strip by a roll-bonding procedure.

8 Claims, No Drawings- METHOD OF MAKING ALUMINIUM BEARING ALLOY STRIP The present invention relates to the production of bearing alloys in strip form, either united to a backing strip of steel or other suitable metal in strip form or in the form of a self-supporting strip for bonding to a backing strip in a subsequent operation.

It is known to employ aluminium, alloyed with substantial quantities of lead, as the anti-friction wearing surface of a bearing. Lead exhibits very low equilibrium solid solution values in aluminium.

Under conventional casting procedures there is a tendency for the lead constituent to be unevenly distributed as a result .of the difference in density of lead and aluminium and for the lead to form a continuous interdendritic network. This continuous network severely weakens the material, reducing tensile strength and fatigue resistance. Some improvement may be effected by working or heating to break up the structure.

Thus we have appreciated that the precipitation of the lead alloying constituent in the form of nonconnected particles uniformly distributed through the aluminium matrix could lead to a significant improvement in the strength and fatigue resistance of the alloy. It is well understood that the particle size of a precipitated alloying constituent is very largely controlled by the rate at which the molten metal is cooled during the casting process. Although rapid quenching is conventionally employed in the known method of producing bearing alloys in shapes suitable for bearing application nevertheless these have not permitted the production of aluminium-lead bearing alloys with a uniform distribution of finely divided lead particles.

A process has been described by A.R.E. Singer in Metals and Materials, June 1970, pages 246-257, for forming metal strip by entraining fine droplets of molten metal in a stream of gas, preferably nitrogen, and projecting the entrained droplets against a moving substrate to form a somewhat porous, but coherent strip, which is then compacted by a hot rolling operation, i.e., rolling at a temperature in the range of 200 to 500C.

We have now appreciated that, with the exceptionally high chill rate experienced by the droplets on striking the target substrate, this method of depositing a metal on a substrate will result in deposition of the lead constituent in the form of a very fine particulate precipitate uniformly deposited in the individual droplets. The subsequent hot working step for compaction of the strip would result in further precipitation of the lead alloying constituent, but would not result in the undesirable continuous network.

Since initially the gas stream moves far more rapidly than the metal droplets, the droplets arecooled by their contact with the gas, so that, in dependence upon the distance between the metal spray nozzle and the target substrate, amongst other factors, the droplets undergo a substantial degree of cooling before striking the target. Under correctly selected conditions the metal strikes the target substrate in the form of undercooled droplets which flatten and almost instantaneously solidify under the high rate of chilling at the target surface.

In accordance with the present invention an aluminium alloy containing l-l% Pb (and optionally other alloying constituents), is produced in strip form by employment of the abovementioned spray casting technique, followed by rolling, without or after separation from the substrate on which it has been deposited. Thus where the sprayed alloy is deposited on a steel strip, which can undergo reduction under the rolling conditions required for the compaction of the aluminium alloy deposited by the spray casting operation, stock material for bearing production may be produced in a single series of operations.

The alloy may contain up to 30 percent, but preferably only up to 5% Sn, up to 12% Si and/or up to about 5% Cd, as is already known for bearing alloys. It may also contain up to about 2% Cu to provide greater strength. However these other additions have little or no effect on the distribution of lead particles in the flattened, solidified droplets of bearing metal deposited on the substrate.

In this method a backing strip of steel (or other material suitable for the purpose) is prepared, preferably by shotor grit-blasting, to bond with the spray-cast metal deposited on it. To ensure a secure bond a thin layer of commercial purity aluminium or other suitable intermediate aluminium alloy is first deposited on the backing strip beforethe layer of bearing alloy is deposited. In other cases it is preferred to make a two-layer stock material which can then be bonded to a suitable steel backing in a subsequent rolling operation. In this way it may be necessary to prepare the surface of the substrate so as to avoid permanent adhesion of the spraycast metal. Since .the adhesion of the bearing alloy is low, this is preferably applied first to the substrate, followed by the layer of commercial purity aluminium (or other material) for forming the intermediate layer of the final bearing. After casting the composite is stripped from the substrate and compacted by rolling.

When stock material is prepared for bonding to a steel backing by a subsequent roll-bonding operation,

the compaction of the spray-cast material is preferably achieved by preceding the hot roll consolidation by cold rolling (between 20-50 percent reduction). It should be added that the hot roll consolidation is still required to break down the droplet boundary films so as to permit inter-droplet welding.

It is well known that aluminium can be bonded to steel by rolling them together under appropriate conditions of temperature and pressure. For this reason, according to a further improvement, the stock material is prepared in the form of a three-ply'sandwich consisting of the bearing alloy coated on both faces by a thin layer of commercial purity aluminium. The commercial purity aluminium may be replaced by any other alloy which will become firmly bonded to the bearing alloy during the course of the process and to a steel backing either in the course of deposition or in a subsequent roll-bonding operation.

The function of the commercial purity aluminium layers are two-fold. First, they greatly assist the hot roll consolidation to be carried out to produce a three-ply alloy. Second, one layer permits maximum adhesion of the alloy to the steel backing strip during subsequent There are added advantages with this procedure. There would be no need to identify the faces of the three-ply strip prior to roll bonding to indicate which side has the commercial purity aluminium, as would be the case with a two-ply system. Also, the amounts of expensive lead (or other alloying constituent) wasted in swarf obtained from the final machining operation would be negligible.

In carrying out the process of the invention the layer of aluminium-lead bearing alloy is deposited under conditions such that, on meeting the substrate, the rate of cooling of the droplets is at least C/sec. Preferably the rate is about 10 C/sec. or even higher. In order to achieve the desired rate of cooling on striking the substrate the droplet size is brought to below 1 mm by the spraying conditions and more preferably the metal is sprayed under such conditions that the droplet size is in the range of 0.1 to 0.2 mm.

Where the object is to produce a backing strip, lined with a bearing alloy, the choice of substrate (material and thickness) must be compatible with the requirements of the bearing backing plate and with the process requirement that it must be capable of further rolling. Annealed mild steel sheet of thickness 0.72 mm (0.028 in.), which was prepared by grit-blasting with chill-cast angular grit to produce a roughened surface, has been used. It is desirable to minimise surface oxidation by avoiding unnecessary delay between this surface preparation and the spray deposition of the intermediate layer (or underlay), followed by the spray deposition of the bearing alloy itself. The choice of the alloy for the underlay is governed by the requirement of obtaining maximum adherence of the final overlay and either commercial purity aluminium or an Al-Si alloy (up to the eutectic composition 12% Si) is suitable.

The spraying conditions are less critical for the underlay than for the overlay of bearing alloy which in one example was an alloy containing 6% Pb, 4% Si, 1.1% Cd, 0.1% Cu, 0.1% Mg and remainder Al. The alloy was sprayed from a temperature of 850C with a nozzle to substrate distance of 350 mm (14 in.). Nitrogen was used as the spraying gas with a measured consumption of 1,120 litres/1 kg (18 cu.ft./l lb.) of metal. These conditions gave the required structure, showing a finely divided second phase, together with retained super-saturation in the final spray deposit and with a cooling rate estimated from the microstructure to be 10 C/sec. It should be emphasized that these precise figures can be varied depending on the design of nozzle, rate of throughput, etc., with the ultimate aim of achieving at the deposit stage the required structure previously described. The thicknesses of the underlay and overlay deposit can be arranged to suit the requirements of the final rolled bearing laminate. In the present case, it was arranged that the final total thickness of both layers after rolling was 0.56 mm (0.022 in.) of which 0.13 mm (0.005 in.) is the intermediate underlay of 99.7 percent aluminium. These thicknesses are those after rolling which is carried out in the temperature range 300-450C using the minimum preheat necessary prior to rolling. The rolling reduction used is that required to break up the deposit structure and to heal porosity and this should not be less than 60 percent reduction of deposit. The effect of hot rolling on the structure is to cause further precipitation of the second phase and growth of existing second phase particles but nevertheless it remains a structure in which the finelydistributed and non-connnecting particles of second phases can have an average size 1 pm.

In other cases it is preferred to treat the surface of the substrate so that the two-layer spray-cast metal may be peeled off the substrate before compaction by rolling. The bearing stock material, thus formed, may be united with aluminised steel strip by roll bonding in conventional manner.

The process of the present invention may also be employed for the production of Al-Sn bearing alloys in separate strip form or united to a steel backing.

We claim:

1. A method of producing aluminium alloy material for the production of bearings comprising the formation of strip material by spraying molten aluminium alloy containing ll0% Pb onto a moving substrate and subsequently compacting the strip of spray-deposited metal, the sprayed metal being entrained in a stream of inert gas under such conditions that the droplets are substantially undercooled before striking the substrate, the cooling rate to completion of solidification after contact with the substrate for the majority of the droplets being at a rate of at least 10 C/sec.

2. A method according to claim 1 in which the aluminium alloy contains up to 12% Si.

3. A method according to claim 1 in which the aluminium bearing alloy is united with a layer of spraydeposited aluminium or aluminium alloy suitable for bonding to a steel backing.

4. A method according to claim 3 in which the bonding layer is deposited on and bonded to a steel backing in the course of deposition and before deposition of the lead-containing bearing alloy.

5. A method according to claim 3 in which the composite strip of lead-containing bearing aluminium alloy and the bonding deposit is separated from the substrate before compaction.

6. A method according to claim 5 in which layers of aluminium or aluminium alloy for bonding to steel are spray-deposited and united to both faces of the layer of spray-deposited lead-containing aluminium alloy before separation of the composite strip from the substrate.

7. A method according to claim 5 in which after separation from the substrate the composite strip is preferably reduced by 20-50 percent by cold-rolling and then subjected to hot compaction.

8. A method of producing bearings in which a composite strip produced by the method of claim 5 is bonded to a steel backing strip by roll-bonding. 

2. A method according to claim 1 in which the aluminium alloy contains up to 12% Si.
 3. A method according to claim 1 in which the aluminium bearing alloy is united with a layer of spray-deposited aluminium or aluminium alloy suitable for bonding to a steel backing.
 4. A method according to claim 3 in which the bonding layer is deposited on and bonded to a steel backing in the course of deposition and before deposition of the lead-containing bearing alloy.
 5. A method according to claim 3 in which the composite strip of lead-containing bearing aluminium alloy and the bonding deposit is separated from the substrate before compaction.
 6. A method according to claim 5 in which layers of aluminium or aluminium alloy for bonding to steel are spray-deposited and united to both faces of the layer of spray-deposited lead-containing aluminium alloy before separation of the composite strip from the substrate.
 7. A method according to claim 5 in which after separation from the substrate the composite strip is preferably reduced by 20-50 percent by cold-rolling and then subjected to hot compaction.
 8. A method of producing bearings in which a composite strip produced by the method of claim 5 is bonded to a steel backing strip by roll-bonding. 